Walking on Sand or Ice

On Ice too little friction: The smooth surface cannot provide the backward reaction force (Newton's 3rd Law) needed to push you forward feet slip.

On Sand surface gives way: Loose particles shift under foot pressure, absorbing energy without returning a solid reaction force. Each step requires 2–3× more energy than firm ground.

Feet hurt on sand because: Calf, arch, and Achilles tendon muscles work 2–3× harder; feet sink and grip sand; constant balance correction strains ankles and knees.

Class 9 Core Concept: Walking requires firm ground reaction (Newton's 3rd Law) + adequate friction both are impaired on sand and ice for opposite reasons (too loose vs. too slippery).